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Author Topic: Whats the problem with reflecting radiation?  (Read 2115 times)

sharkeyandgeorge

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Whats the problem with reflecting radiation?
« on: 24/03/2010 11:30:29 »
Ive been trying to get to the bottom of this but feel I'm overlooking something basic.

I was recently reading an article on x-ray lasers, although the article didn't specify methodology I presume that reflection must play a part in the focusing of the laser, if so why cant we reflect all radiation? would it require matter denser than lead with a high albedo or some sort of repulsively charged surface. Would this if possible be better than using lead and concrete to absorb the radiation?

As I say I feel I'm missing something fundamental which makes this a farcical question, but what is it?


 

Offline techmind

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Whats the problem with reflecting radiation?
« Reply #1 on: 11/12/2010 12:53:17 »
Can I refer you back to this thread:
http://www.thenakedscientists.com/forum/index.php?topic=31260.msg322303;boardseen

In brief, for conventional general-purpose mirror to give specular reflection you need a surface which is reasonably smooth, and electrically conductive, on the scale of the wavelengths you're hoping to reflect. The problem with reflecting X-ray is that the wavelength is so small (comparable to atomic size) that the electrical conduction appears holey. The electric field from the X-ray radiation field cannot be sufficiently 'shorted', and reflection doesn't really work.

To reflect X-rays, instead you use mirrors based on the principle of diffraction. You use a crystal (often silicon) and arrange for the 'reflection' to occur by constructive interference between the weak reflections off the atomic layers. These "mirrors" only work at particular angles of incidence and wavelength. Sometimes you see X-ray focussing mirrors based on this crystal-films deposited on metal surfaces - these normally only work for very glancing-angles of incidence.

I have no idea what kind of mirrors you'd use in an X-ray laser. You can't normally use crystal "mirrors" to reflect the light back at 180 degrees. I don't know whether you could use a succession of "mirrors" to send the X-rays around a ring?


See also
http://www.azom.com/details.asp?ArticleID=741
« Last Edit: 11/12/2010 13:16:26 by techmind »
 

Offline techmind

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Whats the problem with reflecting radiation?
« Reply #2 on: 11/12/2010 12:55:30 »
2) The high electron-density in high-atomic-number metals such as lead facilitates much X-ray absorbtion and scattering, but scattering is incoherent so doesn't result in useful/specular "reflection"
 

Offline maffsolo

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Whats the problem with reflecting radiation?
« Reply #3 on: 11/12/2010 14:00:27 »

 

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